Active region formation through the negative effective magnetic pressure instability

TL;DR

This paper investigates how the negative effective magnetic pressure instability can lead to the formation of active regions on the Sun by causing large-scale magnetic structures through turbulence effects.

Contribution

It demonstrates the connection between the negative effective magnetic pressure instability and active region formation, supported by simulations and stability analysis.

Findings

Instability onset occurs at the same depth in simulations.

Maximum growth rate is independent of magnetic field strength.

Redistribution of turbulence and pressure may produce observable signatures.

Abstract

The negative effective magnetic pressure instability operates on scales encompassing many turbulent eddies and is here discussed in connection with the formation of active regions near the surface layers of the Sun. This instability is related to the negative contribution of turbulence to the mean magnetic pressure that causes the formation of large-scale magnetic structures. For an isothermal layer, direct numerical simulations and mean-field simulations of this phenomenon are shown to agree in many details in that their onset occurs at the same depth. This depth increases with increasing field strength, such that the maximum growth rate of this instability is independent of the field strength, provided the magnetic structures are fully contained within the domain. A linear stability analysis is shown to support this finding. The instability also leads to a redistribution of turbulent intensity and gas pressure that could provide direct observational signatures.